1. Field of the Invention
This invention relates to underwater flowlines for offshore production of oil or gas or the like and more particularly to a method and apparatus for connecting an underwater flowlines to an underwater connection point in water having a substantial current running in substantially a single direction.
2. Brief Description of the Prior Art
A development of offshore oil and gas wells frequently necessitates the use of offshore installations for drilling and subsequently for producing from the completed subsea well. These installations are fixed or floating platforms which are positioned over the well head. It is usually necessary to provide one or more flowlines from the well head or other underwater connection point to the shore or to the platform above or to another platform or subsea well. The flowlines may serve many purposes, the most being to carry off the output of oil or gas from a well. The underwater connection point is often a multiple conduit manifold or connector which is commonly called a "Christmas Tree". The flowlines are generally lowered from a floating support which is often referred to as a lay barge. The flowline is unreeled from the barge or other floating support and lowered to the floor of the ocean. The end of the flowline is connected to a cable and pulled into an alignment funnel where it is connected to the underwater connection point.
When a flowline is being connected to an underwater connection point in water having a substantial current, the flowline drifts substantially away from the connection point and the current forces the flowline itself away. As a result, the flowline is pulled into the connection funnel at an angle by the cable and must undergo a relatively sharp bend which creates stresses in the flowline leading to early failure.
In the laying and connecting of underwater flowlines it is well known to provide buoys or controlled ballasting for controlling the amount of flexing of the flowline in a vertical direction, as in Lochridge U.S. Pat. No. 3,978,677. Sorenson U.S. Pat. No. 3,589,136 discloses an elongated underwater conduit support member for supporting the conduit and controlling its curvature in a vertical direction during the laying of a flowline. Neal U.S. Pat. No. 3,650,114 discloses an apparatus and method for maintaining a controlled curvature of an underwater flowline in a vertical direction to provide for the transition from the connection of the flowline to an underwater connection point to a vertically extended portion which becomes a production riser. None of these patents, however, are concerned with the problem of reducing or eliminating stresses in the lateral non-vertical direction in the flowlines which result from the movement of the flowline or the vessel from which the flowline is fed by water currents.